20-08-2019, 12:52 PM
That burst is very odd.
You may recall that I had to adjust the gain and black level of the raw ADC output. The ADCs in the SAA7118 are 9 bit with the LSB accessible on another pin. Not sure how much difference the extra bit makes but it's certainly used by the decoder part of the chip. 8 bits wre always a touch marginal for digitising an entire composite signal. Professional users have used 10 bit SDI interconnects for many years.
In critical applications, where you want to go from (say) a 10 bit representation to an 8 bit then truncation isn't good enough. The usual method in the broadcast industry is Dynamic Rounding, develope by Quantel. This is reckoned to give better results than dither or other simple rounding methods. I admit I haven't bothered with any clever rounding in the 405 to 625 converter. There's enough noise on the input to provide ample dither.
You may recall that I had to adjust the gain and black level of the raw ADC output. The ADCs in the SAA7118 are 9 bit with the LSB accessible on another pin. Not sure how much difference the extra bit makes but it's certainly used by the decoder part of the chip. 8 bits wre always a touch marginal for digitising an entire composite signal. Professional users have used 10 bit SDI interconnects for many years.
In critical applications, where you want to go from (say) a 10 bit representation to an 8 bit then truncation isn't good enough. The usual method in the broadcast industry is Dynamic Rounding, develope by Quantel. This is reckoned to give better results than dither or other simple rounding methods. I admit I haven't bothered with any clever rounding in the 405 to 625 converter. There's enough noise on the input to provide ample dither.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
